This invention relates to a printhead and a printing apparatus using the printhead, and more particularly, to a printhead which performs printing in accordance with an ink-jet method and a printing apparatus using the printhead.
In ink-jet printing, noise upon printing is very small and negligible, and printing speed is high, further, a print image can be fixed onto a so-called normal paper without special processing. Recently, attention is focused on the ink-jet printing method having these advantages.
Among ink-jet printing methods, a printing method disclosed in Japanese Patent Publication Laid-open No. 54-51837 and DOLS No. 2843064, for example, has a feature different from other ink-jet printing methods in that thermal energy is applied to liquid such as ink so as to obtain a driving force for discharging the liquid.
That is, according to the above printing method disclosed in these publications, printing is performed by causing a state change with sudden volume increase in the liquid acted upon by the thermal energy, then discharging the liquid from an orifice at the end of a printhead by the action based on the state change, as liquid droplets, and attaching the liquid droplets to a print medium.
Especially, according to DOLS No. 2843064, the method is very effectively applied to so-called drop-on-demand printing. Further, the method easily realizes a full-line type printhead having a printing width corresponding to the entire width of a print medium and orifices in a high density. Accordingly, high-resolution and high quality image can be printed at a high speed.
The printhead to which the printing method is applied has orifices to discharge liquid, liquid channels, connected to the orifices, each including a heat action portion to supply thermal energy to liquid, and a substrate having electrothermal transducers (heat generators) to generate the thermal energy.
Recently, the substrate not only holds the plurality of heat generators but also integrates a plurality of drivers to drive the respective heat generators, a logic circuit including a shift register for temporarily storing image data of number of bits corresponding to the number of heat generators, to transfer the image data serially inputted from a printing apparatus to the respective drivers in parallel, a latch circuit which temporarily latches data outputted from the shift register, and the like.
FIG. 16 is a block diagram showing the arrangement of a logic circuit in a conventional printhead having N heat generators (printing elements).
In FIG. 16, reference numeral 400 denotes a circuit board; 401, heat generators; 402, power transistors; 403, an N-bit latch circuit; and 404, an N-bit shift register. Numeral 415 denotes a sensor for monitoring resistance values of the heat generators 401 and the temperature of the circuit board 400 and a heater to maintain the temperature of the circuit board 400. The sensor may be integrated with the heater, or a plurality of sensors and heaters may be packaged. Numerals 405 to 414 and 416 denote input/output pads. Among these input/output pads, the pad 405 is a clock input pad for inputting a clock (CLK) to operate the shift register 404; the pad 406, an image data input pad for serially inputting image data (DATA); the pad 407, a latch input pad for inputting a latch clock (LTCLK) to hold image data in the latch circuit 403; the pad 408, a drive signal input pad for inputting a heat pulse (HEAT) to externally control driving period by turning the power transistors 402 ON to energize the heat generators 401; the pad 409, a drive power input pad for inputting a driving power (3-8V; generally 5V) for the logic circuit; the pad 410, a GND terminal; the pad 411, a heat generator power input pad for inputting power to drive the heat generators 401; the pad 412, a reset input pad for inputting a reset signal (RST) to initialize the latch circuit 403 and the shift register 404; and the pad 413, an HGND terminal for heat generator drive power source.
Further, numerals 414a and 414b denote an output pad for outputting a monitor signal and an input pad for inputting control signals for sensor drive and drive of the temperature maintaining heater. Further, numerals 416-(1) to 416(n) denote block-selection signal input pads for inputting block selection signals (BLK1 to BLKn) for block selection in time-division drive. In time-division drive, the N heat generators are divided into n blocks, and driven in block units. Numeral 417a denotes AND circuits which calculate the logical products of the outputs from the latch circuit 403 and the block selection signals (BLK1 to BLKn); and 417b, AND circuits which calculate the logical products of outputs from the AND circuit 417a and the heat signal (HEAT).
Numerals 418a and 418b denote parasitic resistances which occur on the wiring used for driving the heat generators 401.
The drive sequence of the printhead having the above construction is as follows. In the following description, image data (DATA) is binary data where 1 bit corresponds to 1 pixel.
First, the image data (DATA) is serially outputted from a printing apparatus main body to which the printhead is attached, in synchronization with a clock (CLK), then the data is inputted into the shift register 404. Next, the image data (DATA) is temporarily stored in the latch circuit 403, and ON/OFF outputs in correspondence with image data value (xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) are made from the latch circuit 403.
In this state, when a heat pulse (HEAT) and a block selection signal are inputted, power transistors supplied with ON outputs from the latch circuit 403, corresponding to heat generators in a block selected by the block selection signal, are driven for xe2x80x9cONxe2x80x9d period of the input heat pulse (HEAT). Then, an electric current flows through the corresponding heat generators. Thus, the print operation is performed.
Next, the parasitic resistances 418a and 418b will be described.
It is preferable that the parasitic resistance does not exist, however, actually it cannot be ignored. The example of FIG. 16 shows the parasitic resistances in the logic circuit of the printhead, however, parasitic resistance also exists on a PCB (printed circuit board) within the printhead or a flexible printer cable (FPC) connecting the printhead and the printing apparatus.
In FIG. 16, as the resistances are common to the plurality of heat generators 401, the ratio between the parasitic resistances and the resistance of all the driven heat generators differs dependent on the number of time-divisionally driven heat generators. As a result, the value of a voltage applied to the heat generators (in other words, the value of voltage drop by the parasitic resistances) changes. Accordingly, the voltage applied to both ends of the heat generators changes due to the duty of a pattern to drive the heat generators, which causes variation in energy to the heat generators.
On the other hand, in accordance with the recent tendency of increase in printing speed, a growing number of heat generators are provided in a printhead, and the drive frequency is increasing. In time division drive, the number of simultaneously-driven heat generators is increasing, therefore, the change of voltage drop due to parasitic resistance is not negligible.
Conventionally, some methods to prevent voltage drop have been proposed. One of these methods is to feed-back control a heat pulse (HEAT) to drive heat generators, on the printing apparatus side, so as to change the pulse width based on a pattern for driving the heat generators of the printhead.
More specifically, as shown in FIG. 17A, on the printing apparatus side, a counter 801 counts the number of simultaneously-driven heat generators based on generated image data, then the counted number is stored into a memory 802. A drive pulse generator 803 modulates the pulse width based on the number. Otherwise, as shown in FIG. 17B, the counter 801 provided in the printing apparatus counts the number of bits of serially-transferred image data at each time-division drive, and the drive pulse generator 803 controls the pulse width based on the counted number.
Further, Japanese Patent Publication Laid-open No. 2-508 discloses a technique to count the number of simultaneously-driven heat generators and to control the pulse width.
However, in the conventional art, the shift register and the latch circuit, which have been already provided in the printhead, a circuit to recognize a pattern to drive the heat generators by time-division drive, a counter circuit used for changing the heat pulse width and the like, must be provided on the printing apparatus side. Thus, control on the printing apparatus side is complicated, and the production cost of the apparatus increases.
The complexity of control will be described with reference to FIG. 18.
FIG. 18 shows image data of a character xe2x80x9cHxe2x80x9d represented as a 16xc3x9716 matrix with 16 dots in a printhead scanning direction and 16 dots in direction of nozzle array of the printhead. Generally, image data generated in the printing apparatus main body is sequentially transferred in accordance with the order of numbers allotted to the matrix, from xe2x80x9c1xe2x80x9d to xe2x80x9c256xe2x80x9d, as shown in FIG. 18. However, when the above data is transferred to the printhead, the order of data transfer is changed in accordance with the construction of the printhead, and the processed data is transferred.
That is, in accordance with the number of nozzles and the printing cycle of the printhead, the order of data transfer is rearranged. As shown in FIG. 18, the transfer order in a case where the number of nozzles of the printhead is xe2x80x9c8xe2x80x9d is different from that in a case where the number of nozzles is xe2x80x9c16xe2x80x9d.
Further, as described above, the heat generators of the printhead are time-divisionally driven in one printing cycle. Thus, the control is very complicated since factors to be considered include various numbers of nozzles of the printhead, the number of simultaneously-driven blocks, and the number of simultaneously-driven heat generators based on image data, and these factors must be fed back for modulation of the pulse width to drive the printhead.
The complicated control will be considered with the examples of FIGS. 17A and 17B. In FIG. 17A, calculation processing is complicated since image data to be subjected to counting dynamically changes in accordance with the construction of the printhead such as the number of nozzles, simultaneously-driven blocks and the like, and the change must be considered in counting processing. On the other hand, in FIG. 17B, as the number of simultaneously-driven heat generators in one printing cycle changes in accordance with the construction of the printhead, the process of transfer image data is complicated.
In both cases, the increase in processing load on the printing apparatus main body side cannot be avoided, and in conventional technique nothing could undertake the processing load on the printhead side.
Further, although the printhead and the printing apparatus are separable, and they are separately manufactured, further, the printhead is exchangeable, in the controller of the printing apparatus side, not only data interface with respect to the printhead but also the construction of the printhead must be considered. Thus, development and design of printing apparatus have been very troublesome.
Accordingly, it is an object of the present invention to provide a printhead with a comparatively simple construction, which reduces the cost of the entire system and development load, effectively utilizes an essential constituent devices of a logic circuit of a printhead such as a shift register, while omits control on a printing apparatus side, and performs stable print operation by itself, while suppresses variation of energy to heat generators due to voltage drop by parasitic resistance, and a printing apparatus using the printhead.
According to one aspect of the present invention, the foregoing object is attained by providing a printhead having plural heat generators, a driver which drives the plural heat generators, and a divider which divides the plural heat generators into plural blocks and time-divisionally drives the plural blocks based on an externally-inputted block selection signal, comprising: a counter which counts the number of simultaneously-driven heat generators based on externally-inputted image data and the block selection signal; and a modulator which modulates a pulse width of a drive signal applied to the simultaneously-driven heat generators based on a value obtained from counting by the counter.
Preferably, the modulator has an input pad in which a signal used for modulating the pulse width of the drive signal is inputted. The modulation circuit has various embodiments in accordance with the type of signal inputted into the input pad.
That is, in a case where the printhead further comprises a plurality of input pads, and drive signals having different pulse widths are respectively inputted into the plurality of input pads, it is preferable that the modulator includes: (1) a memory for storing a plurality of threshold values; (2) a comparator which compares the plurality of threshold values stored in the memory with the value obtained from counting by the counter; and (3) a selector which selects one of the plurality of drive signals having different pulse widths in accordance with the result of comparison by the comparator.
Further, in a case where a clock signal used for inputting the image data is inputted into the input pad, it may be arranged such that the modulator includes: (1) a generator which generates a plurality of drive signals having different pulse widths based on the clock signal; (2) a memory for storing a plurality of threshold values; (3) a comparator which compares the plurality of threshold values stored in the memory with the value obtained from counting by the counter; and (4) a selector which selects one of the plurality of drive signals having different pulse widths generated by the generator, in accordance with the result of comparison by the comparator.
Alternatively, the modulator may include: (1) a memory for storing a plurality of threshold values; (2) a comparator which compares the plurality of threshold values stored in the memory with the value obtained from counting by the counter; and (3) a generator which generates a drive signal having an optimum pulse width, based on the clock signal, in accordance with the result of comparison by the comparator.
Further, in a case where the printhead further comprises: an N-bit shift register which inputs the image data; an N-bit latch circuit which latches N-bit image data stored in the N-bit shift register; and N AND circuits which obtain logical products of the N-bit image data outputted from the N-bit latch circuit and the block selection signal, the counter counts the number of simultaneously-driven heat generators based on outputs from the N AND circuits.
It is preferable that outputs to heat generators which are not simultaneously driven in time-division drive are connected to one of common signal lines, and the common signal lines are connected to the counter.
Note that the circuit related to common signal lines has various embodiments.
That is, it may be arranged such that (1) the common signal lines are pulled up, and inverters are provided between the common signal lines and the N AND circuits, or (2) the common signal lines are pulled down, and open-drain or open-collector outputs from the N AND circuits are connected to the common signal line, or (3) the common signal lines are pulled down, and amplifiers are provided between the common signal lines and the N AND circuits, further, open-drain or open-collector outputs from the amplifiers are connected to the common signal lines, or (4) the common signal lines are pulled down, and diode switches are provided between the common signal lines and the N AND circuits, further outputs from the diode switches are connected to the common signal lines, or (5) in addition to the construction (4), a bus terminator is connected to an end of each of the common signal lines.
Further, the number of common signal lines is equal to or more than a maximum number of heat generators simultaneously-driven in the time-division drive, and less than the number of the heat generators.
Further, the counter is preferably an adder, and the adder adds up outputs from the common signal lines.
It is preferable that the modulator performs modulation such that when the number of simultaneously-driven heat generators obtained from counting by the counter is larger, the pulse width of the drive signal is wider, while when the number of simultaneously-driven heat generators obtained from counting by the counter is smaller, the pulse width of the drive signal is narrower.
Preferably, the printhead is an ink-jet printhead which performs printing by discharging ink. In this case, the printhead has an electrothermal transducer which generates thermal energy to be supplied to the ink, to discharge the ink by utilizing the thermal energy.
According to the present invention, the foregoing object is attained by providing a printing apparatus which performs printing by using the printhead having the above construction.
In accordance with the printhead of the present invention as described above, the pulse width of a drive signal applied to heat generators is automatically modulated in the printhead in accordance with the number of simultaneously-driven heat generators which always changes based on image data.
The invention is particularly advantageous since the variation in energy to the heat generators, due to the number of heat generators driven in the printhead and parasitic resistance of the printhead, can be suppressed, thus stable printing operation can be performed.
In this construction, it is not necessary to control the energy to the heat generators so as to reduce the variation in the energy due to parasitic resistance of the printhead on a printing apparatus side, and it is not necessary to provide special circuits on the printing apparatus side. This results in suppressing increase in production cost. Further, as it is not necessary to consider the characteristic of a printhead in development and design of a printing apparatus, the printing apparatus can be developed independently of the printhead.
Further, according to the Invention, as the signal used for modulating the pulse width of the drive signal in the modulation circuit is one of drive signals having different pulse widths or a clock signal used for image data input, it is not necessary to generate specific data on the printing apparatus side. Also, it is not necessary to process the specific data on the printhead side. Thus, the pulse width of the drive signal can be modulated with a simple construction.
Further, according to the present invention, as signal lines used for counting the number of simultaneously-driven heat generators are commonly used, the area of circuit board can be reduced, thus the printhead can be downsized.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.